Mobile communications systems, such as cellular or personal communications services (PCS) systems, are made up of a plurality of cells. Each cell provides a radio communications center in which a mobile unit establishes a call with another mobile unit or a wireline unit connected to a public switched telephone network (PSTN). Each cell includes a radio base station, with each base station connected to a mobile switching center that controls processing of calls between or among mobile units or mobile units and PSTN units. In an alternative arrangement, the base stations may be connected to base station controllers that are in turn connected to the mobile switching center.
One type of protocol used in mobile communications is the code-division multiple access (CDMA) protocol, originally implemented as the IS-95 protocol. CDMA is based on a spread-spectrum technique in which multiple users have access to the same band of carriers. This is accomplished by assigning a unique orthogonal code (referred to as the Walsh code) to each mobile unit. To meet requirements for third generation (3G) wireless communications systems, the CDMA technology has been replaced with the CDMA 2000 family of standards. The standards in the CDMA 2000 family include the IS-2000-1, IS-2000-2, IS-2000-3, IS-2000-4, IS-2000-5, and IS-2000-6 (collectively referred to as the “IS-2000 standard”).
According to IS-2000, a mobile station may discontinue the transmission of traffic channels to preserve power and reduce interference when there is no data to send. Such a feature is referred to as the discontinuous transmission (DTX) mode. The DTX feature is available on fixed-rate traffic channels, such as the dedicated control channel and supplemental channel, in the reverse link between the base station and each mobile unit. The CDMA air-link is based on a forward link and reverse link between base stations and mobile units. The forward link is the link from the base station to the mobile unit, while the reverse link is the link from the mobile unit to the base station. Among the channels used in the reverse link are the dedicated control and supplemental channels, which are employed to carry traffic frames and other signaling from the mobile unit to the base station.
Conventionally, in performing outer-loop power control without DTX, such as in IS-95 CDMA systems, the target ratio of energy per bit to noise spectral density (referred to as the target Eb/No) is adjusted based on whether errors occurred in receiving frames. A target Eb/No may be specified for each mobile unit. The target Eb/No may be increased when the frame error rate (FER) rises above a predetermined threshold and decreased when the FER falls below the predetermined threshold. Alternatively, adjustment of the target Eb/No may be based on occurrence of frame errors rather than the FER crossing a threshold. By coordinating the step sizes of the increases and decreases in the target Eb/No, a target FER can be achieved.
If the transmission of traffic frames is discontinued, however, the conventional technique of controlling the target Eb/No based on the transmission of traffic frames during the period of silence is not possible, even if the receiver in the base station can detect that the transmission of frames has ceased. The reception of traffic frames is no longer available to determine exactly what target Eb/No is required for a given quality of reception. Without a way of adjusting the target Eb/No, reliable transmission of frames may not be possible when the mobile unit exits DTX mode and starts transmitting traffic frames again.
Setting a fixed target Eb/No does not entirely solve the problem, since the Eb/No required for successful transmission of traffic frames may vary during the period of silence, in which case no frames may be successfully received at the end of the silence period and a dropped call may result. The target Eb/No may alternatively be set to a high value to ensure reception at the end of the silence period. Such a high value may be the worst-case value based on the velocity of the mobile unit and the known channel conditions. However, setting a high target Eb/No may result in the transmission of excessive power on the pilot, traffic, and power control channels, which may significantly reduce the power reduction that is supposed to result from the use of DTX.
Thus, an improved method and apparatus for performing power control is needed in a mobile communications system.